A broad field of application for friction gears exists in the textile industry. Such gears greatly simplify the power transmission for many machines. A friction gear is however not capable of operating with large forces. The zone of contact between the two friction wheels is relatively small so that the permissible load is limited. The amount of the pressure required is determined by the tangential force to be transmitted, the friction coefficient and the security factor. Further, the specific pressure in the contact zone for a combination of rubber and metal has a value of only about 0.02 kp/mm.sup.2. This value can however, be exceeded when the pressure time is short whence the work of rubber deformation cannot affect the temperature rise in the material structure of the contact zone to such a great extent. With rubber as one of the friction materials the friction coefficient of the moving surface is much lower in humid atmosphere. For a good power transmission under such conditions the minimum value of the vertical to the contact zone directed pressure force must be double the tangential force between the two friction wheels of the gear.
In the pressure zone of the rubber surface internal processes of material displacement occur with complex hydrodynamic components which the inventor has investigated, (see his paper: "To the law of mixed friction" Technical University Munich, Germany).
For automatic regulation of the pressure the driving wheel together with the motor is normally pivoted against the driven wheel which remains fixed. An inferior pressure force in the friction zone is sufficient for power transmission. With increasing load the driving wheel is forced against the driven one, so that increasing power consumption can be transmitted in the friction contact zone. Friction gears do not normally provide for a change of the sense of rotation because then the automatic pressure regulation is not more effective. The uncoupled state of the gear is obtained by displacing the driving wheel from the driven wheel or disengaging intermediate elements between the two friction wheels.
The present invention combines the friction gear with a drive by a flexible tensile force tractive element such as a belt or rope to provide for further uses thereof and practical possibilities independent of the tractive tensile force elements such as a rope being finite or continuous. The present invention also provides a gear having power transmission by friction contact between friction wheels and a drive for said wheels solely by tensile force means in which the disadvantages of the prior art are eliminated. The present invention also provides for automatic pressure regulation in the friction zones of the gear in both directions of rotation thereof. The present invention further provides a gear having a flexible tractive element such as a rope in which the functions of stopping in the final stages and overload conditions are automatically controlled. The gear construction is compact and does not need any significant service.